1. Field of the Invention
The present invention relates to a tunnel junction sensor with an antiferromagnetic (AFM) coupled flux guide and, more particularly, to such a sensor wherein first and second antiferromagnetic layers pin first and second portions of the flux guide at an ABS leaving a highly permeable middle portion which responds to signal fields.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a tunnel junction sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically nonconductive tunneling or barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The tunnel junction sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the tunnel junction sensor for conducting a sense current therethrough. The sense current is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor where the sense current is conducted parallel to the major film planes (CIP) of the spin valve sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the tunnel junction sensor to the sense current (Is) is at a minimum and when their magnetic moments are antiparallel the resistance of the tunnel junction sensor to the sense current (Is) is at a maximum. Changes in resistance of the tunnel junction sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the sense current (Is) is conducted through the tunnel junction sensor, resistance changes, due to signal fields from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the tunnel junction sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the tunnel junction sensor at minimum resistance. The dr/R of a tunnel junction sensor can be on the order of 40% as compared to 10% for a spin valve sensor.
The first and second shield layers may engage the bottom and the top respectively of the tunnel junction sensor so that the first and second shield layers serve as leads for conducting the sense current Is through the tunnel junction sensor perpendicular to the major planes of the layers of the tunnel junction sensor. The tunnel junction sensor has first and second side surfaces which are normal to the ABS. First and second hard bias layers abut the first and second side surfaces respectively of the tunnel junction sensor for longitudinally biasing the magnetic domains of the free layer. This longitudinal biasing maintains the magnetic moment of the free layer parallel to the ABS when the read head is in the quiescent condition.
Magnetic head assemblies, wherein each magnetic head assembly includes a read head and a write head combination, are constructed in rows and columns on a wafer. After completion at the wafer level, the wafer is diced into rows of magnetic head assemblies and each row is lapped by a grinding process to lap the row to a predetermined air bearing surface (ABS). In a typical tunnel junction read head all of the layers are exposed at the ABS, namely first edges of each of the first shield layer, the seed layer, the free layer, the barrier layer, the pinned layer, the pinning layer and the second shield layer. Opposite edges of these layers are recessed in the head. The barrier layer is a very thin layer, on the order of 20 xc3x85, which places the free and pinned layers very close to one another at the ABS. When a row of magnetic head assemblies is lapped there is a high risk of magnetic material from the free and pinned layers being smeared across the ABS to cause a short therebetween. Accordingly, there is a strong-felt need to construct magnetic head assemblies with tunnel junction heads without the risk of shorting between the free and pinned layers at the ABS due to lapping.
A scheme for preventing shorts across the barrier layer of the tunnel junction sensor is to recess the tunnel junction sensor within the head and provide a flux guide between the ABS and the sensor for guiding flux signals from the rotating magnetic disk. Typically, the ferromagnetic material of the flux guide is required to be stabilized by hard bias layers on each side of the flux guide. With track widths of 1 xcexcm or more this stabilization of the flux guide has been acceptable. However, with submicron track widths, such as 0.1 xcexcm to 0.2 xcexcm, the hard biasing of the flux guide renders the magnetization of the flux guide too stiff to adequately respond to flux signals from the rotating magnetic disk. The reason for this is because flux guides, regardless of the track width, are magnetically stiffened about 0.1 xcexcm on each side of the flux guide by the hard biasing layers. When the track width is above 1 xcexcm, this does not render the flux guide unacceptable since a remainder of the width of the flux guide remains relatively soft for responding to field signals from the rotating magnetic disk. Another way of stating the problem is that with submicron track widths the hard bias renders the flux guide with low permeability. Since a flux guide needs a height of approximately 0.25 xcexcm to 0.5 xcexcm the field signal from the rotating magnetic disk is nonexistent or insignificant at the tunnel junction sensor because of the lack of permeability of the flux guide. Accordingly, there is a strong-felt need to provide a submicron track width tunnel junction sensor with a flux guide that has high permeability.
In the present invention the tunnel junction sensor has front and back surfaces which are recessed from the ABS and the flux guide has a ferromagnetic flux guide body with front and back surfaces wherein the front surface forms a portion of the ABS and the back surface is magnetically coupled to the tunnel junction sensor. The flux guide body has first and second side portions and a middle portion with the middle portion being located between the first and second side portions. First and second antiferromagnetic layers are provided wherein the first antiferromagnetic layer is magnetically coupled to the first side portion and the second antiferromagnetic layer is magnetically coupled to the second side portion for pinning magnetic moments of the first and second side portions so that only a magnetic moment of the middle portion of the flux guide defines a track width of the sensor and responds to field signals from a rotating magnetic disk. The first and second antiferromagnetic layers have side walls which are perpendicular to the ABS and are spaced apart by the track width. This is important so that when the head is lapped at the ABS the track width remains constant. Each of the front surface of the tunnel junction and the back surface of the flux guide has a width as measured parallel to the ABS and the major planes of the layers that is greater than the track width. In this manner the tunnel junction may be made significantly larger so as to have less resistance to the tunneling current (IT). Further, the present invention provides a middle portion of the flux guide body which is highly permeable so as to respond to signal fields from the rotating magnetic disk.
In a first embodiment of the present invention, first and second antiferromagnetic layers interface the first and second side portions of the flux guide body. In a second embodiment of the invention, first and second ferromagnetic layers are provided. A first antiparallel coupling layer is also provided and located between and interfacing each of the first ferromagnetic layers and the first side portion of the flux guide body so as to antiparallel couple the first ferromagnetic layer and the first portion of the flux guide body. A second antiparallel coupling layer is also provided between and interfacing each of the second ferromagnetic layer and the second side portion of the flux guide body so as to antiparallel couple the second ferromagnetic layer with the second side portion of the flux guide body. A first antiferromagnetic layer is exchange coupled to the first ferromagnetic layer for pinning a magnetic moment of the first ferromagnetic layer parallel to the ABS and to major planes of the layers so that the magnetic moment of the first side portion of the flux guide is pinned antiparallel thereto. A second antiferromagnetic layer is exchange coupled to the second ferromagnetic layer for pinning a magnetic moment of the second ferromagnetic layer parallel to the ABS and to the major planes of the layers so that the magnetic moment of the second side portion of the flux guide is pinned antiparallel thereto. Accordingly, by an antiparallel coupling for each of the first and second side portions of the flux guide the magnetic moments of the first and second side portions are strongly pinned parallel to the ABS and to the major planes of the layers. This strong pinning is an improvement over even the first embodiment discussed hereinabove. A still further embodiment includes first and second hard bias layers interfacing the first and second side portions of the flux guide respectively for biasing the magnetic moments of the first and second side portions in the same direction as the pinned directions of the magnetic moments of the first and second side portions of the flux guide.
An object of the present invention is to provide a flux guide for a tunnel junction sensor wherein the flux guide can be lapped at the ABS without changing a track width.
Another object is to provide a flux guide with a highly permeable narrow track width at the ABS and a wider portion recessed in the head and magnetically coupled to a tunnel junction sensor which is also wider than the track width.
A further object is to provide a pinning of first and second side portions of a flux guide body which pinning is supported by hard bias layers on each side of the flux guide body wherein the flux guide body has a wide portion recessed in the head which is wider than the track width and which is magnetically coupled to a tunnel junction sensor which is also wider than the track width.
Still another object is to provide various methods for making the above flux guides and tunnel junction sensors.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.